Buggy code:
public abstract class RealVector { public abstract int getDimension ( ) { } public abstract double getEntry ( int index ) { } public abstract void setEntry ( int index , double value ) { } public void addToEntry ( int index , double increment ) { } public abstract RealVector append ( RealVector v ) { } public abstract RealVector append ( double d ) { } public abstract RealVector getSubVector ( int index , int n ) { } public abstract void setSubVector ( int index , RealVector v ) { } public abstract boolean isNaN ( ) { } public abstract boolean isInfinite ( ) { } protected void checkVectorDimensions ( RealVector v ) { } protected void checkVectorDimensions ( int n ) { } protected void checkIndex ( final int index ) { } protected void checkIndices ( final int start , final int end ) { } public RealVector add ( RealVector v ) { } public RealVector subtract ( RealVector v ) { } public RealVector mapAdd ( double d ) { } public RealVector mapAddToSelf ( double d ) { } public abstract RealVector copy ( ) { } public double dotProduct ( RealVector v ) { } public double cosine ( RealVector v ) { } @ Deprecated public abstract RealVector ebeDivide ( RealVector v ) { } @ Deprecated public abstract RealVector ebeMultiply ( RealVector v ) { } public double getDistance ( RealVector v ) { } public double getNorm ( ) { } public double getL1Norm ( ) { } public double getLInfNorm ( ) { } public double getL1Distance ( RealVector v ) { } public double getLInfDistance ( RealVector v ) { } public int getMinIndex ( ) { } public double getMinValue ( ) { } public int getMaxIndex ( ) { } public double getMaxValue ( ) { } public RealVector mapMultiply ( double d ) { } public RealVector mapMultiplyToSelf ( double d ) { } public RealVector mapSubtract ( double d ) { } public RealVector mapSubtractToSelf ( double d ) { } public RealVector mapDivide ( double d ) { } public RealVector mapDivideToSelf ( double d ) { } public RealMatrix outerProduct ( RealVector v ) { } public RealVector projection ( final RealVector v ) { } public void set ( double value ) { } public double [ ] toArray ( ) { } public RealVector unitVector ( ) { } public void unitize ( ) { } public Iterator < RealVector . Entry > sparseIterator ( ) { } public Iterator < RealVector . Entry > iterator ( ) { } public RealVector map ( UnivariateFunction function ) { } public RealVector mapToSelf ( UnivariateFunction function ) { } public RealVector combine ( double a , double b , RealVector y ) { } public RealVector combineToSelf ( double a , double b , RealVector y ) { } public double walkInDefaultOrder ( final RealVectorPreservingVisitor visitor ) { } public double walkInDefaultOrder ( final RealVectorPreservingVisitor visitor , final int start , final int end ) { } public double walkInOptimizedOrder ( final RealVectorPreservingVisitor visitor ) { } public double walkInOptimizedOrder ( final RealVectorPreservingVisitor visitor , final int start , final int end ) { } public double walkInDefaultOrder ( final RealVectorChangingVisitor visitor ) { } public double walkInDefaultOrder ( final RealVectorChangingVisitor visitor , final int start , final int end ) { } public double walkInOptimizedOrder ( final RealVectorChangingVisitor visitor ) { } public double walkInOptimizedOrder ( final RealVectorChangingVisitor visitor , final int start , final int end ) { } protected class Entry { private int index ; public Entry ( ) { } public double getValue ( ) { } public void setValue ( double value ) { } public int getIndex ( ) { } public void setIndex ( int index ) { } } @ Override <START_BUG> public boolean equals ( Object obj ) { <END_BUG> throw new UnsupportedOperationException ( ) ; } @ Override public int hashCode ( ) { } protected class SparseEntryIterator implements Iterator < RealVector . Entry > { private final int dim ; private RealVector . Entry current ; private RealVector . Entry next ; protected SparseEntryIterator ( ) { } protected void advance ( RealVector . Entry e ) { } public boolean hasNext ( ) { } public RealVector . Entry next ( ) { } public void remove ( ) { } } public static RealVector unmodifiableRealVector ( final RealVector v ) { } }

Fix:
public boolean equals ( Object other ) {